The present invention relates to a method for heating a blast furnace stove for use with a blast furnace.
The combustion air supplied to a blast furnace is typically preheated using a stove, comprising refractory material which is heated using a burner. When the material is hot enough, combustion air is passed through the stoves to pre-heat it before injection into the blast furnace. Usually, several stoves are operated in parallel and cyclically so that at least one stove is operated for heating combustion air while the refractory material of at least one stove is heated.
Conventionally, the top gas leaving the blast furnace has a temperature of around 110-120° C. and contains about 20-25% each of CO and CO2. Typically, 3-5% H2 and some H2O will also be present, but the other major constituent of the top gas is N2 (typically 45-57%). The gas constitutes a low grade fuel, having a relatively low heating value, and is commonly used to fuel the stoves.
The top gas is normally combusted using air-fuel burners in the stoves. In order to ensure the necessary high air blast temperatures needed by the blast furnace, it is known to enrich the top gas with a high calorific value gas, such as coke oven gas or natural gas. The combustion of such additional fuel leads to larger overall emissions of carbon dioxide from the plant, and is therefore not desirable.
It is also known to oxygen enrich the combustion air used in stack burners. Usually, the enrichment levels needed to reduce or eliminate the need for additional, high-calorific fuels are such as to result in a final oxidant oxygen content in the combustion air of around 28-30%.
Such methods may in some cases render peak flame temperatures high enough to damage the refractory material of the stove, and it may be necessary for example to supply an excess air rate to suppress the flame temperature.
It is further known to pre-heat, using heat recovery units, the fuel and air fed to the stove burners.
All the above-described methods add complexity to the process and require costly equipment.
The blast furnace itself is a highly efficient counter-current reactor that has evolved over many years. It is approaching the limits of thermodynamic efficiency, which is why it is difficult to reduce energy consumption relative to current best operating practices. Moreover, the blast furnace and its ancillary equipment, such as stoves, are the largest energy consumers in an integrated iron and steel works. Furthermore, the energy consumed in iron making is the dominant factor determining the carbon consumption of the integrated steel making process, and therefore the emissions of carbon dioxide. Therefore, it would be desirable to increase thermal efficiency of blast furnace stoves.
Using so-called “carbon capture” techniques, it is possible to separate carbon dioxide from the stove flue gas, in order to lessen emissions. However, such separation is relatively expensive. Therefore, it would be desirable to design a blast furnace stove allowing cheaper carbon capture.
In addition to the problem of high peak temperatures mentioned above, too low flame temperatures or heat input rates will lead to long heating cycles, which is undesirable. In other words, the flame temperature needs to be moderated.
The present invention solves the above described problems.